Chemical Imaging

FT-IR Spectroscopy & Imaging

FT-IR and FT-NIR take advantage of the functional groups of every molecule that generate characteristic absorption or transmission spectra that definitively identifies that chemical compound. Both FT-IR and FT-NIR images give rich information revealing the identity of a broad range of chemical components in materials, as well as displaying areas of homogeneity and difference. In this example a solid dose form tablet of paracetamol sold over the counter under the Panadol brand was examined. A typical use of FT-IR is compound identification as shown in Fig. 1 below where measured spectra are compared to library or reference spectra. There is good agreement between the measured and reference spectra even though in this case, given the tablet is not pure paracetamol, there are spectral overtones from other constituent substances.

Fig. 1: The FT-IR spectrum of a sample taken from the tablet (black curve) compared to a reference spectrum for paracetamol (green).

Going beyond comparing bulk spectra as above we can carry out FT-IR imaging. Such an image consists of a 2-D array of measured spectra which means that the distribution of the chemical composition can be investigated. Principle Component Analysis (PCA) of a sample can identify the major absorptive ingredients and also identify their distribution in the sample. Uses include verifying sample purity and authenticity as well as analysing contaminants in raw materials and end product. In the example images below both the total absorption map and a PCA map of the same tablet region are shown. If the component of interest has strong local absorption in a defined spectral region then an absorption map centred on this spectral region of interest can provide most if not all of the required information. For more complex samples or ones with unknown components then PCA may be required to extract the required information and show the component distribution.

Fig. 2: Left is a typical absolute absorption map of the Panadol tablet while right is the corresponding PCA map showing the chemical distribution.

Raman Spectroscopy & Mapping

Every compound has its own unique Raman spectrum, providing a virtual fingerprint for identification. Identifying, characterizing and investigating the structures of a wide range of material types can be easily achieved with little preparation. The following shows an over the counter Panadol Extra tablet. The major difference between the composition of the standard Panadol tablet tested using FT-IR above is that the Extra tablet also contains caffeine. Fig. 3 below shows typical Dispersive Raman spectra, one for paracetamol and one for caffeine. For both components a unique identifiable spectral peak can be resolved, allowing us to distinguish between paracetamol and caffeine.

Fig.3: Dispersive Raman spectra of Caffeine (blue) and Paracetamol (red) with the starred bands at 555cm-1 and 690cm-1 unique to each substance.

As with the FT-IR we can go beyond comparing bulk spectra as above and can carry out Raman imaging. However the Raman technique does not lend itself to area scan with images instead being constructed from a point map of the sample, with a spectrum collected at each position. The images of the Panadol Extra tablet below are constructed in this manner in order to investigate the distribution of paracetamol and caffeine. Once the map with all spectral data has been collected, the unique defined spectral peaks can be used to construct the images seen below. The paracetamol distribution is found to be quite homogeneous, which is to be expected for the Active Pharmaceutical Ingredient (API) in a well blended tablet. A uniform distribution, as in this case, should lead to a controlled linear release of the API during dissolution. On the other hand the caffeine distribution is clustered and random suggesting that the manner of its release does not impact the product efficacy. A final composite image combines the two distributions to give a true indication of the blending process. Imaging in this manner may be used, among other things, to detect counterfeit pharmaceuticals, monitor processes and products and provide data for root cause analysis.

Visible Image
This is a visual image of the tablet under test, which in fact is made up of several images as the microscope field of view is restricted.

Paracetamol Image
In this map the intensity of the Paracetamol peak at 690cm-1 is displayed. This is a false colour image with blue indicating low intensity and red high.

Caffeine Image
In this map the intensity of the caffeine peak at 555cm-1 is displayed. As above this is a false colour map.

Combined Image
In this map the distributions of the Paracetamol and caffeine are overlapped to give a single image. The areas of Paracetamol are shown as red, caffeine as blue and neither as black.